account_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
	<data>
	
		<!-- Invoice line Sum  -->
	    <record id="view_invoice_line_tree_inherit" model="ir.ui.view">
            <field name="name">account.invoice.line.tree.inherit</field>
            <field name="model">account.invoice.line</field>
            <field name="inherit_id" ref="account.view_invoice_line_tree"/>
            <field name="arch" type="xml">
	            <field name="quantity" position="attributes">
	            	<attribute name="sum">Quantity</attribute>
	            </field>
            </field>
        </record>
	
    	<!-- Sum for supplier invoice  -->
    	<record id="invoice_supplier_form_inherit" model="ir.ui.view">
            <field name="name">account.invoice.supplier.form.inherit</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_supplier_form"/>
            <field name="priority">20</field>
            <field name="arch" type="xml">
	            <xpath expr="//field[@name='invoice_line']/tree/field[@name='quantity']" position="attributes">
                    <attribute name="sum">Quantity</attribute>
            	</xpath>
            </field>
        </record>

    	<record id="invoice_form_inherit" model="ir.ui.view">
            <field name="name">account.invoice.form.inherit</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
            <field name="priority">20</field>
            <field name="arch" type="xml">
	            <xpath expr="//field[@name='invoice_line']/tree/field[@name='quantity']" position="attributes">
                    <attribute name="sum">Quantity</attribute>
            	</xpath>
            </field>
        </record>
    	
	</data>
</openerp>